1. Field of the Invention
The present invention relates to a method of establishing in a signal receiving unit a clearly defined digital value of a significant bit position, and on the basis hereof identifying the synchronization of received bit positions and generating therefrom synchronized transferred data packets or data cells occurring as received information-carrying signals, wherein the signal receiving unit is included in a separate signal transfer system.
The invention is based on the occurrence of a bit position that has the logic value "0" or the logic value "1" at each time section or time position. Accordingly, reference is made in the following description solely to "bit position".
A signal transfer system in which the present invention can be applied will include a transmission unit which transmits information-carrying signals that are digitalized through the medium of the values of said bit positions, a signal transfer medium, and a signal receiving unit which receives the information-carrying digitalized signals.
The signal transmission unit is adapted in a known manner to transmit digital information coordinated to form a data packet, and to transmit data packet on data packet or data cell on data cell in an ATM-system with a predetermined number of bit positions in each data packet.
In the following description only "data packet" is referred to.
The following description distinguishes between two categories of "data packet", where a first category of data packet is considered to be a "standardized" data packet and includes a predetermined number of standardized bit positions coordinated to form a number of bytes (data word) and containing address information and/or information-carrying data.
When reference is made in tie following description to "information-carrying" bit positions, these may be address-related or information-carrying.
The present invention is intended more particularly for use in signal transfer systems of the kind which use significant line codes, and even more particularly to systems of the kind in which the line codes are comprised of one bit position (or more bit positions) where the bit positions have one and the same digital value, for instance a logic "0", and occur periodically at time intervals of equal lengths, or a number of bit positions between line-code related bit positions.
More specifically, the system is also based on the assumption that the significant properties of a line code lie in a bit shift which occurs periodically with a predetermined number of information-carrying bit positions therebetween.
This bit shift can be achieved by allocating two bit positions to the line code, i.e. the value "0" and an adjacent value "1", or by assigning solely one bit position to the line code, e.g. the value "0", although in this latter case the system will require a transmitter-related scrambling unit and a receiver-related descrambling unit. (Systematic bit distortion and regeneration).
A data packet thus supplemented with line-code related bit positions forms the second category and is referred to as "a line-code supplemented" data packet.
Although the line code can seize two or more bit positions the following description is based on the assumption that the line code exists as a single bit position, and although the line code can assume one of two digital values, the following description is based on the assumption that the bit position has been allocated the digital value "0".
It is known by the person skilled in this art that the digital value of a bit position can be given the same information content, irrespective of whether the value is "0" or "1", meaning that in the following description of the features of the invention each reference to a digital value "0" can be replaced with the value "1", and vice versa.
In a signal transfer system of the aforesaid kind, the signal receiving unit is adapted to receive digital information which is coordinated to form a data packet and which includes line-code related bit positions.
The signal receiving unit is provided with and/or coacts with first means for establishing a bit shift between two bit positions for each of a number of line-code related bit positions, and a second means for removing solely said line-code related bit positions and, in relation to their time positions, time-synchronize the data flow and further transmit "standardized" data packets.
2. Description of the Background Art
Several different designs of signal transfer systems which include a transmitter unit which transmits information-carrying digitalized signals, a transfer medium for transferring said signals, and a signal receiving unit for receiving transmitted information-carrying digitalized signals are known to the art.
The present invention relates more particularly to a signal transfer system of the kind in which the signal transmitting unit is adapted to transmit information-carrying data packets and to transmit data packet on data packet.
A predetermined number of line-code related bit positions are introduced to and in each "standardized" packet via the transmitting unit, so as to form a "line-code supplemented" data packet.
A system of this kind requires a predetermined value for each selected line-code related bit position (such as "0" or "1"), the selection of a number of coordinated line-code related bit positions (one or two, or more in exceptional circumstances), selection of the time distance (the number of information-carrying bit positions) between each line-code related bit position so that these will be equal to and adapted directly in dependence on (or independently of) chosen data packet limits.
It will be assumed in the following description that such a line-code supplemented data packet will be comprised of a number of subsections where a first subsection may have twenty four bit positions belonging to the information-carrying data packets with digital values corresponding to the information content, followed by a line-code related bit position assigned the digital value "0", followed by a subsection which also has twenty four information-carrying bit positions, followed by a line-code related bit position, and so on, so as to form from a number of complete subsections at least one data packet of the line-code supplemented kind, such data packets normally occuring in series.
In signal transmission systems of the aforesaid kind line-code related bit positions are introduced periodically into the transmission unit, and the signal-receiving unit includes means capable of synchronizing with the bit stream and capable of synchronizing the data packets by evaluating the line-code positions, and removing solely said line-code related bit positions so that only information-carrying digital signals will be forwarded from the signal-transmitting system as a standardized data packet.
It is also known that the transmission properties of the medium used can be improved by inserting line-code related bit positions.
Because the line-code related bit positions appear periodically and have one and the same digital value, and because the separate bit positions are predictable when transmitted, the information-carrying digitalized signals form a periodic serial structure which enables transmission properties to be improved by improving reception conditions in the receiver.
Reference is made to the publication "Digital Telephony" Second edition, pages 165-179 "Digital Transmission and multiplexing" by John Bellamy (ISBN 0-471-62056-4) as an example of present day technology.
Particular reference is made to pages 171, 172, which disclose provisions for "Bit insertion", which is the technique used by the present invention.
Various methods and devices have been proposed for establishing the bit-positions of a line code.
It is also known to structure each line-code supplemented data packet received in the receiver into subsections which each include a number of similar bit positions, wherein each subsection includes a chosen number of information-carrying bit positions and one bit position, or several bit positions, as a line code. The present invention is also based on this technique.
This method of structurizing and storing a large number of subsections, namely a number of such magnitude as to ensure very definitely that each information-carrying bit position in a subsection will have been allocated a digital value ("1") which differs from the chosen line-code value ("0") at least once, enables the time position of the line code to be evaluated by reading the bit positions of each subsection and proceeding on the assumption that the line code will be represented by the sole sequential bit position whose value has not changed. Structuring of the subsections in this way will mean that inserted line codes will occur in the same time positions in each subsection.
Received bit positions and data packets can then be synchronized to the time positions of the line code and the standardized data packet can be forwarded with the bit positions and the data packets synchronized in relation to a synchronizing signal.
Alternatively, each incoming bit position can be counted with the aid of a bit position counter while establishing whether or not similarly located bit positions in each subsection have the same digital value as the selected digital value of a selected line code in a predetermined number of subsections.
If it can be established that similarly located bit positions of a chosen number of subsections have the same values as the line code, it can be assumed that the time position of the line code has been established.
If such is not the case, the bit position counter will jump one step and repeat the searching procedure until all bit positions of the line code and their time positions have been found.
When considering the present state of the art as described above and when taking into consideration that the signal receiving unit used must be provided with and/or cooperate with first means so as to establish a time position between or related to two time positions of a number of line-code related bit positions, and second means for removing solely said line-code related bit positions and, in relation to their time positions, time-synchronized "standardized" data packets and their associated bit positions, it will be seen when considering the necessity of dividing the information-carrying and line-code supplemented signals or data packets received by the signal receiving unit into a number of different subsections a problem exists in evaluating the digital value of each information-carrying bit position so as to enable the bit positions of a line coded to be evaluated very quickly.
It will also be seen that a technical problem resides in providing conditions such that the bit positions of the line code will be apparent immediately the information-carrying bit positions in a time section or time position in evaluated subsections has been allocated a digital value ("1") which differs from a chosen line-code value ("0").
Another technical problem resides in providing conditions which enable the bit positions of the line code to be evaluated without needing to overdimension the choice of the number of subsections, in the aforesaid "very definite" concept.
A technical problem also lies in realizing the significance of and the advantages afforded by allowing the digital value of each evaluated information-carrying bit position to influence a 1-bit register that can be set to "0" or "1", and to include this re-settable 1-bit register in a ring-coupled shift register, with a number of 1-bit registers corresponding to the number of time positions or bit positions in said subsections.
It will also be seen that a technical problem is one of realizing the significance of setting all 1-bit registers in the ring-coupled shift register to an initial setting such that all said 1-bit registers will adopt a digital value that corresponds to the digital value of the chosen line code, such as "0".
A technical problem associated herewith resides in realizing the significance of permitting respective bit positions in the information-carrying signals differing from the digital value of a chosen line code to reset the adjustable 1-bit register to a state which differs from the line code, such as to "1", or vice versa, at a time point which corresponds to the relevant time position of the bit position.
It will also be seen that a technical problem resides in realizing the significance of choosing the number of 1-bit registers to correspond to the number of bit positions in a subsection structured with a corresponding number of information-carrying bit positions and to the number of bit positions in a used line-code, and with a number of re-settable 1-bit registers that corresponds to the number of incoming conductors.
In the case of a shift register of this kind where the digital content of a 1-bit register is shifted to an adjacent 1-bit register between each bit position, with the number of re-settable 1-bit registers corresponding to the number of incoming conductors, a technical problem resides in realizing the advantages that are afforded when it can be concluded that when the shift register shifts solely one line-code related bit position around the ring this sole bit position is representative of the line code.
Another technical problem resides in realizing the significance of and the advantages afforded by the use of a control circuit which is able to read the bit-position values in said ring-coupled shift register and also to point out the bit position which alone carries a line-code related value and whose time position can therefore be assumed to be representative of the line-code position in each subsequent subsection.
Another technical problem resides in realizing the advantages that are afforded by using a control circuit which can be controlled to establish the line-code bit positions and to establish therefrom a time position which corresponds to a synchronous reference and activate a synchronizing unit and therewith produce a synchronized time position related to the line code position.
It will also be seen that a technical problem resides in realizing the significance of permitting the control circuit to activate means for removing the line code positions when the line-code supplemented data packets received by the signal receiving unit are to be forwarded to another signal receiving unit as standardized signals and data packets, i.e. signals and data packets that are not line-code supplemented.
A further technical problem resides in creating a ring-coupled shift register for the purpose intended, wherewith the functional principles of the shift register can be utilized both in the serial transmission of bit positions and in the parallel transmission of bit positions, constructing data packets having line-code related bit positions.